The invention relates to an apparatus for correctly registering mounting of a printing plate on a plate cylinder of a sheet fed offset press.
An apparatus with electrically isolated registering pins at the front plate clamping device is known from Gennan utility model No. 77 28 905. The registering pins are connected with an electrical source and a signal transmitter. An electrical circuit is closed and the signal transmitter is activated when the registering pins contact the printing plate in the registering holes.
It is a disadvantage that the signal transmitter is also activated if the printing plate is not correctly registered. This is for instance the case when deformations or too large tolerances in the registering hole are causing skewing of the printing plate. The signal transmitter is also activated if, for instance, the printing plate is laterally misaligned so that the registering pins are touching the front edge of the printing plate. Is also possible when using a U-shaped printing plate which has registering holes open at its front edge when the registering pins are in contact with a side of the holes, without establishing a correct registration at the bottom of the openings.
The apparatus shown in European patent No. 581 212 B1 eliminates these disadvantages. The registering pins are also electrically isolated and connected to an electrical source and a signal transmitter. The registering pins have an additional partial isolation on their surface so that only a certain conducting section detects the bottom part of the registering hole.
It is a disadvantage of the apparatus that the registering pins are costly to manufacture. The insulated surface carries high loads and therefore either the surface or the entire registering pin has to be made from ceramics. This material cannot withstand the abrupt load changes that can occur, and these can cause breakage of the registering pin.
It is an object of the invention is to provide an apparatus to control the correctly registered position of a printing plate, which can be realized with simple means and can withstand abrupt load changes of the printing plate.
This object is solved according the invention by inserts allocated to registering elements, which are contacting the printing plate electrically isolated from the registering elements and are connected to a control unit. The plate cylinder has in the plate cylinder gap a plate clamping apparatus of a front plate clamping device and a rear plate clamping and tensions device. Registering elements for the lateral and circumferential direction are located in the front plate clamping device. These registering elements coincide with the registering punches in the printing plate and are connected to a control unit.
It is a particular advantage of the present invention that it employs a wear resistant and fracture-proof registering element which can be manufactured with low cost and guaranteeing a secure operation.